The present invention is directed, in general, to voltage controlled resistors and, more specifically, to voltage controlled resistors with high resistance-to-voltage linearity for use in tuning circuits and the like.
Voltage controlled resistors, having a resistance which varies with an applied voltage, have use in a wide variety of applications, including, for example, tuning circuits. Metal oxide semiconductor (MOS) transistors may function as voltage controlled resistors if operated in the ohmic region, with the gate-source voltage controlling the resistance.
Within the signal path of a signal processing circuit, however, the resistance of an MOS transistor employed to provide voltage controlled resistance also changes with the source-drain voltage. If the gate voltage is held constant, the resistance of the transistor changes with the source voltage, introducing high non-linearity in the resistive behavior of the transistor. This non-linearity becomes higher as the overdrive voltage applied to the transistorxe2x80x94the gate-source voltage minus the threshold voltage (Vgsxe2x88x92Vt)xe2x80x94decreases. As the voltage supply becomes lower, providing a good overdrive of the transistor becomes increasingly difficult, particularly if the source (assuming an n-channel transistor) cannot be connected to ground.
There is, therefore, a need in the art for a voltage controlled resistor having a high linearity of resistance per unit change in applied voltage.
To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in an integrated circuit, a voltage-controlled resistance structure formed from a number of voltage-controlled resistance cells, each including a transistor with a biasing capacitor connected between the gate and source and an associated controller coupled to the capacitor to maintain a steady charge on the biasing capacitor and keep the gate-source voltage at a control voltage corresponding to a desired resistance. The gate voltage applied to each transistor is able to xe2x80x9cfloatxe2x80x9d together with the source voltage in order to keep the gate-source voltage constant, and the resistance structure exhibits improved voltage-dependent resistance linearity together with a larger range of biasing while lowering needed refresh frequencies to avoid noise injection.
The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art will appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.
Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, it may be advantageous to set forth definitions of certain words or phrases used throughout this patent document: the terms xe2x80x9cincludexe2x80x9d and xe2x80x9ccomprise,xe2x80x9d as well as derivatives thereof, mean inclusion without limitation; the term xe2x80x9corxe2x80x9d is inclusive, meaning and/or; the phrases xe2x80x9cassociated withxe2x80x9d and xe2x80x9cassociated therewith,xe2x80x9d as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like; and the term xe2x80x9ccontrollerxe2x80x9d means any device, system or part thereof that controls at least one operation, whether such a device is implemented in hardware, firmware, software or some combination of at least two of the same. It should be noted that the functionality associated with any particular controller may be centralized or distributed, whether locally or remotely. Definitions for certain words and phrases are provided throughout this patent document, and those of ordinary skill in the art will understand that such definitions apply in many, if not most, instances to prior as well as future uses of such defined words and phrases.